In a cellular system of most interest to this invention the channel from a base station to the mobile station is referred to as the forward channel, i.e., the forward control channel and the forward voice channel. The channel from the mobile station to the base station is referred to as the reverse channel, i.e., the reverse control channel and the reverse voice channel. The system may have one or more forward control channels defined as combined paging and access channels on which the mobile station receives page messages, indicating that an incoming call has been received for the mobile station, and system access parameters needed to access the cellular system.
In conventional practice (e.g., TIA/EIA/IS-91 (AMPS)), and when entering a system access task, the mobile station scans its receiver to identify the two strongest received access channels. The mobile station then registers into the paging area having the strongest access channel, from which it can read overhead information sent on a forward access channel. If the mobile station cannot read and decode the data successfully, it switches to the second strongest channel and tries to complete the task there.
During an Initialization Task, and referring to FIG. 3A, in accordance with TIA/EIA/IS-91, the mobile station waits for an overhead message from a base station in two separate sub-tasks, specifically an Update Overhead Information Task (TIA/EIA/IS-91, section 2.6.1.1.2) and a Verify Overhead Information Task (section 2.6.1.2.2). Both of these sub-tasks can last up to three seconds.
During a System Access Task, and referring to FIG. 4A, the mobile station executes four sub-tasks, specifically a Retrieve Access Attempt Parameters Task (section 2.6.3.3, which can last up to 1.1 seconds), an Update Overhead Information Task (section 2.6.3.4, which can last up to 1.5 seconds), an Await Message Task (section 2.6.3.8, which can last up to 10 seconds for a Base Station Challenge and up to 5 seconds for other access types), and an Await Registration Confirmation Task (section, 2.6.3.9, which can last up to 5 seconds).
As is indicated in FIGS. 3A and 4A, during both the Initialization and System Access Tasks, respectively, the mobile station may receive a page message on the paging/access channel. However, as currently specified the mobile station ignores any page messages that are received at this time. That is, the mobile station does not respond to a received page message until after the completion of the Initialization or System Access Tasks, that is until entering an Idle Task. Only at this time is the mobile station enabled to respond to an incoming call.
As can be appreciated, a significant period of time can elapse before the mobile station enters the Idle Task and can respond to a next page message (e.g., up to 20+ seconds when in the System Access Task). As a result, the mobile station may not be able to respond to a page message, resulting in a missed call.